Since resonators including a piezoelectric body can be miniaturized by using a semiconductor process or the like, in recent years, the application areas thereof including a power generating element, various types of actuators, and the like, have been extended.
FIG. 24 shows one example of a sensor module placed in power supply line 110 in which power generating element 101 is used as a driving power source.
Power generating element 101 includes beam part 102 and piezoelectric body 104. Piezoelectric body 104 is provided on a main surface of beam part 102 and sandwiched between upper electrode 105 and lower electrode 103. Weight part 106 made of a permanent magnet is attached to an end portion of beam part 102. Beam part 102 is vibrated by interaction with a magnetic field change, which is caused by an AC current flowing in power supply line 110, in the vicinity of power supply line 110.
Hereinafter, beam part 102 together with upper electrode 105, lower electrode 103, and piezoelectric body 104 are defined as a beam. Actually, not only beam part 102 but also a whole beam including upper electrode 105, lower electrode 103, and piezoelectric body 104 is vibrated.
Distortion generated by the vibration of the beam generates electric charges proportional to pressure by a piezoelectric effect of piezoelectric body 104, thus generating a voltage. Since the generated voltage has a size proportion to an amount of an electric current flowing in power supply line 110, control unit 109, which monitors the voltage generated in piezoelectric body 104 and wirelessly transmits the monitored results, serves also as a current sensor. Electric power necessary for storage, processing, transmission, or the like of sensor information in control unit 109 is covered by electric power generated by power generating element 101, thus achieving a sensor module that does not need to be supplied with electric power from a battery or outside.
Note here that prior art literature information on the invention of the present application includes, for example, Patent Literature 1 and Non-Patent Literature 1.
However, the above-mentioned power generating element needs to match an alternating-current frequency of power supply line 110 and a resonance frequency of power generating element 101 with each other in order to obtain maximum generating efficiency. However, the resonance frequency tends to vary due to variation in manufacturing processes because the resonance frequency depends upon a length, a thickness, a weight of weight part 106 attached to the end portion of beam part 102, and the like.